Seminal smell on culture is produced by?

Culture of proteus bacilli have a characteristic putrefactive odour described as seminal / fishy. Discrete colonies are seen in young culture but thereafter actively motile cells spread on the surface of the plate in successive waves to form a thin filmy layer in concentric circles. Swarming growth is a problem in laboratory when mixed growth is obtained in which proteus bacilli are present with other bacteria. Several methods have been used to inhibit swarming which includes, increase concentration of agar, incorporation of chloral hydrate, sodium azide, alcohol, sulphonamide, surface active agents / boric acid. Swarming does not occur on MacConkey agar on which smooth colourless colonies are formed. Ref: Ananthanarayanan & Paniker's microbiology 8th Ed Page 280.